Methodology
How the calculators work, and where they stop
Every number this site produces comes from a model we publish here, with a version, a date and a verification status. This page is the audit trail.
Last reviewed 2026-11-10 · Roof Repair Roof editorial team
The principle
A planning tool should be checkable. That means showing the inputs, the arithmetic, the assumptions and the bounds — and stating clearly when the numbers are not yet verified. We would rather publish a labelled provisional band than an unlabelled confident one.
The repair cost model
The model prices the amount of repair, never the size of the house. Each category has a unit — leak locations, linear feet of flashing or metal detail, square feet of shingle, patch or affected area — a rate band in US dollars per unit, and a minimum job charge.
The arithmetic is: quantity × rate band, multiplied by a material factor where the category has one, then by an access factor and, if selected, a temporary-protection factor; then raised to the category's minimum charge if the result is lower. The low end is rounded down and the high end up to the nearest $25, so the arithmetic result always stays inside the published band. The midpoint is the average of the two ends, labelled as a scenario midpoint rather than a typical price.
| Adjustment | Value | Basis |
|---|---|---|
| Straightforward access | ×1.00 | Editorial assumption, single storey and walkable slope |
| Moderate access | ×1.15 | Editorial assumption, two storeys or obstacles |
| Difficult access | ×1.35 | Editorial assumption, steep, fragile or restricted site |
| Temporary / out-of-hours protection | ×1.35 | Editorial assumption for temporary weatherproofing and premium scheduling |
| Quantity not known | Band widened ±25% | Stated assumption instead of a missing input |
| Material not known | Band widened ±10% | Mid-band material factor plus visible uncertainty |
| Access not known | Band widened ±10% | Moderate factor plus visible uncertainty |
| Rounding | Low down, high up to $25 | Deliberate: keeps the result inside the published band |
When you cannot answer a question, the calculator does not quietly invent an answer. It applies a stated assumption — the typical quantity for the category, the moderate access factor, a mid-band material factor — and widens the band by 25% for an unknown quantity, 10% for an unknown material and 10% for unknown access. Each assumption is printed under the result, because a wide honest band is more useful than a narrow invented one.
No regional multipliers are applied. Regional pricing requires licensed local data, and we do not have it, so we do not manufacture a state or city index.
What validating the model would require
- A licensed dataset of recent US repair invoices, with scope, quantity and region, or a documented sample collected with consent.
- A written review by a practising roofing professional or a construction-cost analyst, recorded with a name and a date on this page.
- Recalculation of every rate band against that data, with the source, unit, applicable scope and access date recorded per line.
- A published revision history, so a visitor can see which version produced their result.
- A scheduled re-review, at least annually, with the review date shown in the calculator.
Until those steps are complete, the model stays labelled provisional and its outputs stay out of any claim that they represent current market prices.
The roof area model
Plan area is length × width. The slope factor is √(1 + (rise ÷ 12)²), where rise is measured per 12 units of horizontal run. Roof surface is plan area × slope factor. One roofing square is 100 square feet. A material allowance is a separate, user-controlled percentage and is never folded into the geometric area.
The approximation assumes a single rectangular gable or shed plane. It does not account for hips, valleys, dormers, multiple planes, or the difference between a footprint and a roof edge. Where measurements exclude the overhang, the tool shows a separate adjusted figure assuming one foot of overhang per side, and labels that assumption.
Verified examples: a 40 ft × 30 ft footprint at 0/12 gives 1,200 sq ft; at 6/12 it gives approximately 1,341.6 sq ft, because the slope factor is 1.1180. Metric and imperial conversions use 1 sq ft = 0.09290304 m².
The quote comparison
The comparison tool performs arithmetic on values you type in: totals, absolute and percentage differences against a basis you select, and consistency checks across scope, area, material and four inclusion fields. It stores nothing on a server, sends nothing to analytics, and does not score, rank or recommend contractors.
Its warnings are statements about your inputs, not about the contractors. “Disposal is not stated in Quote B” means exactly that — it is not an accusation.
Sourcing and dates
Where a page states something we cannot observe ourselves, it links a source near the claim and lists it at the end with an access date. Sources are chosen for relevance to the specific point, not for volume. Our own explanations are labelled as ours.
Publication and update dates reflect a real review of the page. We do not advance a date to make a page look fresh, and we do not publish artificial word-count targets.